Is atmospheric spraying viable against global warming?

New York, Nov 23 A novel programme designed to reduce Earth's heat capture by injecting aerosols into the atmosphere from high-altitude aircraft "is possible", but unreasonably costly, suggests a study.

Researchers from the Harvard University explored the capabilities and costs of various methods of delivering sulphates into the lower stratosphere, known as stratospheric aerosol injection (SAI).

By deploying material to altitudes of around 20 kms, the hypothetical "solar geoengineering" project beginning 15 years from now would aim to halve the increase in anthropogenic radiative forcing.

"Solar geoengineering is often described as 'fast, cheap, and imperfect'," said Gernot Wagner, from Harvard's John A. Paulson School of Engineering and Applied Sciences.

"While we don't make any judgement about the desirability of SAI, we do show that a hypothetical deployment programme starting 15 years from now, while both highly uncertain and ambitious, would be technically possible strictly from an engineering perspective," he added. 

In addition, the team stressed that modified existing planes cannot do the job, entirely new plane design is needed, which could also be remarkably inexpensive, "at an average of around $2 to 2.5 billion per year". 

"It would indeed take an entirely new plane design to do SAI under reasonable albeit entirely hypothetical parameters. No existing aircraft has the combination of altitude and payload capabilities required," explained co-author Wake Smith, a lecturer at Yale College.

Further, the study, published in the journal Environmental Research Letters, also developed the specifications needed for SAI with direct input from several aerospace and engine companies. 

The new plane needs to be "equivalent in weight to a large narrow body passenger aircraft. But to sustain level flight at 20 kms, it (also) needs roughly double the wing area of an equivalently sized airliner, and double the thrust, with four engines instead of two," Smith said.

"At the same time, its fuselage would be stubby and narrow, sized to accommodate a heavy but dense mass of molten sulphur rather than the large volume of space and air required for passengers."

The team estimated the total development costs at less than $2 billion for the airframe, and a further $350 million for modifying existing low-bypass engines.

The new planes would comprise a fleet of eight in the first year, rising to a fleet of just under 100 within 15 years. The fleet would fly just over 4,000 missions a year in year one, rising to just over 60,000 per year by year 15.

"Given the potential benefits of halving average projected increases in radiative forcing from a particular date onward, these numbers invoke the 'incredible economics' of solar geoengineering. Dozens of countries could fund such a programme, and the required technology is not particularly exotic," Wagner said.